1. The Field of the Invention
The present invention relates to a mobile communication system for carrying out multiple access using spread spectra, and particularly to a neighboring cell search method applied to handover control during communication or to zone re-selection control during the idle mode in the system and a mobile station constituting the system.
2. The Relevant Technology
A mobile communication system like a widespread mobile phone system offers its services by dividing the entire service area into rather small radio zones called cells. As shown in FIG. 1, such a system comprises a plurality of base stations 111 for covering divided radio zones (cells), and mobile stations 112 for communicating with the base stations by establishing radio channels.
Direct Sequence CDMA (DS-CDMA) is a scheme for a plurality of users to carry out communications using the same radio frequency band by transmitting information through second modulation that spreads a conventional information data modulation signal with a high rate spreading code. The radio signal of each user is identified by a spreading code assigned to the user.
In the mobile communication system, the spreading code used for the spreading usually consists of a combination of two types of spreading codes: a “first spreading code group” with the same period as an information symbol period and commonly assigned to all the base stations; and a “second spreading code” with a considerably longer period than the information symbol period and uniquely assigned to each of the base stations.
FIG. 2 is a schematic diagram illustrating a method of using the spreading codes in the mobile communication system to which the present invention is applied. In FIG. 2, the upper layer represents a scrambling code layer 202 with a long period uniquely assigned to individual base stations, and the lower layer represents a channelization code layer 204 with a short period commonly assigned to all the base stations. The signals transmitted from the base stations are identified using long period scrambling codes uniquely assigned to the individual base stations. A plurality of codes redefined as the scrambling codes for the entire system, and system designers select the codes to be assigned to the base stations among them.
For the mobile stations to demodulate information transmitted from the base stations, they must receive the information in synchronization with the timing of the spreading code repeated periodically at the transmitting side. In particular, as for the scrambling codes, detection of the timing requires a long time because of the long period. Accordingly, it is important for the mobile station to detect the repetition timing of the scrambling codes to demodulate perch channels of the base stations. In the present specification, the repetition timing of the scrambling codes is referred to as “phase”. It is not necessary to detect the absolute phase in practice, but to find the relative difference in the timing between the scrambling codes of the base stations, that is, the phase difference. Thus, the term “phase” refers to the relative phase between the scrambling codes in the present specification.
FIG. 3 is a schematic diagram illustrating timing relationships between the scrambling codes associated with signals sent from the base stations to a mobile station.
FIG. 3 illustrates a case of an inter-cell asynchronous mobile communication system, in which synchronization between the base stations are not necessarily required, and the timing of the scrambling codes received by the mobile station differs for each base station. On the contrary, in an inter-cell synchronous system establishing synchronization between the base stations, the timing of the scrambling codes is exactly adjusted to the timing assigned in advance to the base stations. Accordingly, the relative timing of the scrambling codes between the base stations is fixed and unchangeable. Comparing the inter-cell asynchronous system with the inter-cell synchronous system, the former has an advantage over the latter that it does not require any timing source such as the GPS (Global Positioning System) which is necessary for the synchronous system, and hence is more flexible in extending the system or the like.
The radio signal transmitted from a base station at certain transmission power travels through space with a certain attenuation, and arrives at a receiving site. Since the attenuation the radio signal undergoes increases with the distance from the transmitting site to the receiving site, it is common that a perch channel transmitted from a distant base station is received at a lower received level, and a perch channel transmitted from a near base station is received at a higher received level. In practice, however, the propagation loss is not determined only by the distant, but varies because of such conditions as the geography and buildings. As a result, the received power of the perch channels from the base stations fluctuate sharply with the move of the mobile station. In the condition in which the received levels of the perch channels from the base stations fluctuate sharply, perch channels received above a certain required received level alter incessantly. This is because the received level of the current perch drops suddenly, or the received level of a perch un-receivable increases abruptly above the receivable level. Thus, to receive the signals from the base stations with better quality, it is important for the mobile station to continuously monitor the perches from the base stations, and to select the best base station.
In the asynchronous mobile communication system, a mobile station must search for a perch quickly whose spreading code and phase are unknown. As a method of searching for a phase, there is one called “3-step cell search” disclosed in a document by K. Higuchi, M. Sawahashi and F. Adachi, “Fast Cell Search Algorithm In Inter-Cell Asynchronous DS-CDMA Mobile Radio”, IEICE Trans. Commun., Vol. E81-B, No.7, July 1998. The method provides a “masked symbol” part of the perch channel which undergoes double spreading by a channelization code and a scrambling code. Here, the “masked symbol” is spread only by the channelization code without using the scrambling code.
FIG. 4 is a schematic diagram illustrating a structure of a perch channel.
First, the mobile station despreads the received signal using a channelization code 404 commonly used by all the base stations. This enables the mobile station to detect a peak at the timing of a masked symbol 408 of received signal independently of the types of the scrambling codes (first step).
Subsequently, in response to the timing extracted at the first step, the mobile station detects a scrambling code group code 406 superimposed at the same position as the masked symbol 408, and identifies the group to which the scrambling code belongs which is used by the base station in connection with the reception (second step).
Finally, using the scrambling codes belonging to the group determined at the second step, the mobile station identifies the scrambling code 402 used by the base station (third step).
In the system to which this method is applied, a lot of scrambling codes are divided into groups in advance. In contrast, in the inter-cell synchronous system, since the phase differences of the scrambling codes between the base stations are known in advance, and hence the searching timing can be limited to a fixed timing width (search window), the power consumption or time taken for the cell search can be saved.
The conventional search method in the inter-cell asynchronous system, however, requires more power consumption and time for the cell search than the inter-cell synchronous system, presenting a problem of exhausting the battery power of the mobile terminal quickly. On the other hand, employing the inter-cell synchronous system to simplify the cell search of the mobile station presets problems of hindering making full use of the above mentioned advantages of the inter-cell asynchronous system, and of increasing the cost of the total system.
As described above, the mobile communication system such as a currently wide spread mobile phone system comprises the plurality of base stations 111 for covering divided radio zones, and the mobile stations 112 for communicating with the base stations by establishing radio channels as shown in FIG. 1.
The radio signal transmitted from a base station at certain transmission power travels through space with a certain attenuation, and arrives at a receiving site. Since the attenuation the radio signal undergoes increases with the distance from the transmitting site to the receiving site, a perch channel transmitted from a distant base station is usually received at a lower received level, and a perch channel transmitted from a near base station at a higher received level. In practice, however, the propagation loss is not determined only by the distant, but varies because of such conditions as the geography and buildings. As a result, the received power of the perch channels from the base stations fluctuates sharply with the move of the mobile station. Thus, to receive the signals from the base stations at better quality, it is important for the mobile station to continuously monitor the perch channels from the base stations, and to select the best base station. To select the best base station, the mobile station must continuously confirm the propagation condition of a captured perch channel, or search for uncaptured new perch. Such confirmation of the propagation state of the captured perch channel and the search for the uncaptured new perch channel are generically called “quality measurement of the perch channel” the present specification.
On the other hand, a technique called intermittent reception is applied to the mobile station to prolong the life of the battery by reducing the power consumption. Although the mobile station in an idle mode must continuously monitor the paging, the intermittent reception halts the receiver as much as possible when unnecessary to receive, thereby saving the power consumption.
FIG. 5 is a schematic diagram illustrating a structure of a paging channel defined by ARIB IMT-2000 Study Committee, “Japan's Revised Proposal for Candidate Radio Transmission Technology on IMT-2000: W-CDMA Revised Proposal Version 1.1” (September, 1998, ARIB). According to this paper, to increase the effect of the intermittent reception, the paging channel is structured such that multiple mobile stations are divided into a plurality of groups, and paging signals for respective groups are mapped onto a single physical channel. FIG. 5 illustrates a paging signal assigned to one of the groups. In FIG. 5, reference symbols PIs each designate a very short signal informing whether paging is present or not. Reference symbols MUIs each designate a portion including paging information (ID number of mobile station). In FIG. 5, such a configuration is assumed in which the PIs are transmitted twice (PI1 and PI2) to improve the receiving accuracy of the PI, and four pieces of paging information (MUI1-MUI4) can be transmitted for four mobile stations per group. In other words, the paging signal consists of two PIs and four MUIs, and receiving time period per paging signal is about 15 milliseconds. The paging channel consists of multiplexed paging signals with the same structure, the number of which equals the number of the groups. FIG. 5 illustrates that the mobile station receives the paging signal of its own group at every 720 millisecond interval.
The mobile station receives the PI portion, first, and then the MUI portion only when a decision is made that the paging is present as a result of receiving the PI portion. This offers two advantages: First, it is enough for the mobile station to receive only the paging of its own group; and second, to receive only the PI portion when there is not paging information. This in turn can limit the actually required receiving time rate to a small value, making it possible to reduce the power consumption to a very small amount.
FIG. 5 illustrates the paging information which is already transmitted from the base station and selected through the decision by the mobile station. In an actual situation, however, the mobile station must search for the perch channels of the neighboring base stations as it moves. Since the mobile station must receive a lot of receivable perch channels to search for the neighboring base stations, it is important to minimize the frequency of the search operation to increase the effect of the intermittent reception.
Thus, to select the best base station with the movement of the mobile station, there is a tradeoff between the continuous monitoring of the perch channels of the neighboring base stations by searching and receiving them, and the reduction in the operation time rate of the receiver to prolong the battery of the mobile station as long as possible. On the one hand, the reduction in the operation time rate of the receiver will results in a decrease in the selection accuracy of the base station, bringing about undesirable results such as degradation in the service quality. On the other hand, an increase in the operation time rate of the receiver to improve the selection accuracy of the base station will consume the battery of the mobile station quickly, presenting a problem of markedly impairing the usefulness of the mobile station. In the conventional cell search control method, however, the quality measurement of the perch channel is implemented periodically as described in the following paper with considering the tradeoff between the selection accuracy of the base station and the service quality: K. Yunoki, A. Higashi and N. Tsutsumi, “Cell Search Strategy on W-CDMA Mobile Station”, B-5-186 of the 1999 IEICE General Conference. Specifically, since the quality measurement of the perch channel is carried out independently of the reception of the paging signal, the mobile station must operate its receiver at both timings of quality measurement of the perch channel and the reception of the paging signal, which presents a problem of increasing the consumption of the battery, one of the essential resources of the mobile station.